FBI Director Robert Mueller testifies during a hearing before the Senate Judiciary Committee on June 19. / Alex Wong, Getty Images

by Kevin Johnson, USA TODAY

by Kevin Johnson, USA TODAY

WASHINGTON - FBI Director Robert Mueller acknowledged Wednesday that the agency has deployed drones to conduct surveillance in the U.S., and that the bureau was developing guidelines for their future law enforcement use.

Mueller told the Senate Judiciary Committee that the unmanned aerial vehicles, whose use by law enforcement has raised questions from privacy advocates and civil liberties groups, are deployed in "a very minimal way and very seldom.''

"Our footprint is very small,'' the director said. "We have very few.''

Responding to questions posed by lawmakers, including Sen. Dianne Feinstein, D-Calif., Mueller said he would provide additional information to the panel about how information and images collected during the surveillance operations are used and stored.

But he asserted drone use was "narrowly focused on particular cases and particular needs.''

Iowa Sen. Chuck Grassley, the panel's ranking Republican, initiated the drone discussion Wednesday, saying that Attorney General Eric Holder--in previous committee testimony--said that the Drug Enforcement Administration and Bureau of Alcohol Tobacco Firearms and Explosives had "purchased drones and were exploring their use.''

"Absent from (Holder's) response was an indication of how the FBI was using or is seeking to utilize drone technology,'' Grassley said.

Chris Calabrese, the ACLU's legislative counsel on privacy issues, said Wednesday that it was "a little troubling'' that the bureau had begun using the technology as guidelines were still being developed.

"If you are going to use potentially invasive surveillance, it seems like you should have the standards in advance,'' Calabrese said.

Sen. Mark Udall, D-Colo., also expressed "concern'' that the FBI was only in the initial stages of "developing privacy guidelines that protect civil liberties.''

"Unmanned aerial systems have the potential to more efficiently and effectively perform law enforcement duties,'' Udall said, "but the American people expect the FBI and other government agencies to first and foremost protect their constitutional rights.''

In a separate written statement following the director's testimony, the FBI said drone use is limited to learning "critical information that otherwise would be difficult to obtain without introducing serious risk to law enforcement personnel.''

Earlier this year, the bureau deployed a drone to assist in its investigation of an Alabama man who had taken a 5-year-old boy hostage and barricaded himself and the boy in an underground bunker.

"FBI's UAVs (unmanned aerial vehicles) are only utilized to conduct surveillance operations on stationary subjects,'' according to the bureau statement. "In each instance, the FBI must first obtain the approval of the Federal Aviation Administration to use the aircraft in a very confined geographic area.''

Questions about the bureau's use of the controversial tool came as Mueller defended the National Security Agency's secret phone and Internet tracking programs recently disclosed by former defense contractor Edward Snowden.

Mueller, in what is expected to be his final appearance at the committee before his term expires in September, said both programs were vital to national security.

"Communications are the soft underbelly of terrorist (operations),'' Mueller said. "If that goes dark on us, we will be sitting waiting for the next attack.''

On Tuesday, NSA Director Keith Alexander told the House Intelligence Committee that the programs have helped thwart more than 50 terrorist threats worldwide, including more than 10 targeting the U.S.